The overrunning clutch is tuned by design to stay engaged just a little more of the amount of inertia decay of normal throttle changes. Abrupt or total loss will disengage, but normal flight throttling does not. Flat pitch rotors retains a lot of inertia, but with with flight , there is a lot of drag on the blades because the angles of attack to fly and maneuver. That keeps the engine and blades married. That’s the problem with low inertia blade systems used on R22s and homebuilt helicopters. The drag is way higher than the inertia, and they will decay so fast with out action that they are essentially stopped because they are out of useful rpm. Flat pitch you can split the engine and blades easily. Once flying, the blades are trying to stop everything from turning and only the engine pushing the blades spinning is keeping it in the air. One to two seconds is the reaction time to get in autorotation mode or you may be in trouble. That’s a lot of drag over inertia.